After a few drinks, the advice is always “drink some water”. But how much, exactly? Here are the numbers for beer, wine and spirits, where they come from, and how Hydra uses them to tell you how much water offsets an evening out.
Why alcohol takes water away
Your body uses a hormone to tell the kidneys to hold on to water. Alcohol lowers it, so the kidneys let more water go and you pass more urine than you drank. A classic study measured the effect at about 10 ml of extra urine for every gram of alcohol.
That makes the strength of the drink the key number:
- a 500 ml beer at 5% has about 20 g of alcohol;
- a 150 ml glass of wine at 12% has about 14 g;
- a 50 ml shot of spirits at 40% has about 16 g.
A beer is mostly water, so even after the alcohol it still gives you some water back. Wine about breaks even. A shot has very little water and almost as much alcohol as a beer, so it takes away more than it brings. Coffee, by contrast, doesn’t dehydrate you at all in normal amounts.
The numbers, drink by drink
In Hydra, alcohol has a daily allowance of 20 g, about one 500 ml beer. Within it, each drink counts with its own factor, so the water in it still counts. Past it, the water no longer counts, only the alcohol does. I explain how the daily alcohol limit works in more detail on the support page.
| Drink | First of the day | Past 20 g |
|---|---|---|
| Beer, 500 ml (5%) | +300 ml | −200 ml |
| Beer, 330 ml (5%) | +198 ml | −132 ml |
| Wine, 150 ml (12%) | 0 ml | −150 ml |
| Spirits, 50 ml (40%) | −110 ml | −160 ml |
So after a first beer, every further drink asks for about a glass of water: 130–200 ml per drink. Two 500 ml beers leave you about 200 ml short. Add a shot of spirits and it’s about 350 ml.
Why I chose the cautious estimate
The 10 ml per gram figure comes from an older study, and newer ones found a smaller effect for weak drinks. In a study with older men, beer didn’t raise urine output, while spirits did for a few hours. In the beverage hydration index study, a 4% lager was kept about as well as water over four hours.
When I built Hydra, I had to pick one number. I went with the cautious one on purpose. If it’s a little too strict, you drink a glass of water you didn’t strictly need. If it were too generous, Hydra would tell you you’re fine when you aren’t. The 20 g allowance is there for the same reason: a first beer still counts as some water, but you can’t reach your daily goal by drinking beer.
How much water to drink, in practice
- Have a glass of water with every drink after the first.
- Spirits ask for the most: about 150–160 ml of water per shot.
- Finish the evening with one more glass, before you go to bed.
- Water replaces the fluid alcohol takes away; it doesn’t undo alcohol’s other effects.
To see the exact amount for your own evening, add your drinks to the drinks calculator: it uses the same rules as the app. And if you want the whole picture of what each drink adds, it’s all built on net hydration.
Questions
Does alcohol dehydrate you?
Yes. Alcohol makes your body lose about 10 ml of extra water for every gram of alcohol, which is why strong drinks take away more water than they bring.
Does beer hydrate you?
A little. A 500 ml beer is mostly water, so the first one of the day still adds about 300 ml in Hydra. Past about 20 g of alcohol, each further beer takes about 200 ml away.
Is wine dehydrating?
Wine at 12% about breaks even on its own. Past the first drinks of the day, each 150 ml glass takes about 150 ml of water away.
How much water should I drink per shot?
About 150–160 ml. A 50 ml shot of spirits at 40% has about 16 g of alcohol and very little water.
How much water should I drink after two beers?
About 200 ml. The first 500 ml beer still hydrates you a little; the second takes about 200 ml away.
Does drinking water prevent a hangover?
Not on its own. Water replaces the fluid alcohol makes you lose, but it doesn’t undo alcohol’s other effects.
Do electrolyte drinks help after alcohol?
They’re kept by the body even better than water, so they count more in Hydra (×1.5). Water works too; it just takes a little more of it.
Is 20 g of alcohol a day a safe amount?
No, that’s not what it means. In Hydra, 20 g is only the point after which the water in alcoholic drinks stops counting. It isn’t a recommendation about how much alcohol to drink.
